How to store constants that can be read from all program, and also be used in defining functions?

Question

I need to form a file named "constants.txt", and then use them when defining runge Funtion. Since the constants in the main function cannot be used outside the main function. How can I do about it?

Thanks!

float runge(float x, float y){
   return (-a*x+b*y+c/4.0+e*delta_t);} // where a,b,c,d,e,delta_t are from the file created
int main(){
    ofstream vOut("constants.txt", ios::out | ios::trunc);
    vOut.precision(5);
    vOut << "1.22" 
            << setw(5) <<"9.81"
            << setw(4) <<"0.0"<<endl;
    vOut << "3.0"  
            << setw(4) <<"1.0" 
            << setw(6) <<"0.01"<<endl;  
    vOut.close();
    
    const float a, b, c;
    const float d, e, delta_t;
    
    //read the file and set variables
    ifstream vMyFile("constants.txt");
    int i = 0;
    while (vMyFile >> a >> b >> c >> d >> e >> delta_t) {
    }
            vMyFile.close();

Answer 1

I think the simplest way is to define a header file named like constants.h, which contains like

#ifndef CONSTANTS_H
#define CONSTANTS_H

static const float a = 1;
static const float b = 2;
static const float c = 3;
static const float d = 4;
static const float e = 5;
static const float delta_t = 1E-5;

#endif

Any cpp file can include the header and there is no worry about redifining these variables.